This invention relates generally to deformation sensors and, more particularly, to techniques for measuring overall deformation in a structure that is subject to vibration or shape control using local deformation actuators. Piezoelectric devices are useful in controlling the shape and vibration of mechanical structures, such as in truss or bending members of large mechanical structures in space, in panels, or in large adaptive optical structures.
If a structural element is deliberately deformed by a finite number of deformation actuators, the strain distributions due to these local actuators will be highly non-uniform. There will be virtual strain discontinuities at the edges of the deformation actuators. For example, consider the case of a cantilever beam deformed by bending near its supported end, using a single actuator. The deformation will be concentrated at one small region and the remaining portions of the beam will show little or no local strain. Clearly, any locally positioned sensor will be sensitive only to the strain in the local region, but what is usually of more interest is the strain in a more global sense. The overall or average deformation is of interest because, in most vibration and shape control applications, there is a need to sense the effect of both structural stresses and the stresses induced by actuators, and to control the actuators accordingly to achieve a desired result, such as a desired shape or a reduction in vibrational motion.
One approach to measuring the end-to-end elongation of an axial member is to run an optical fiber along or through the member and to measure its length change using laser interferometry or reflectometry techniques. Free-field optical sensors could also be used instead of fiber optic sensors. For instance, optical laser interferometers can be employed inside hollow tubes to measure length changes from a light source at one end to a reflective target at the other end. The principal problems associated with these optical devices are their cost, large size, large weight, power consumption, limited dynamic range, complexity, and susceptibility to damage from mechanical shock. A further problem with fiber optical sensors is their high sensitivity to temperature changes.
Another technique for obtaining a global measure of deformation is to average the signals derived from a large number of local strain sensors. The difficulty with this approach is that it requires a large number of electrical connections to power and monitor the sensors. Complexity and reliability are major design issues in such an arrangement.
It will be appreciated from the foregoing that there is a need for a simplified approach for estimating the end-to-end or global deformation of a structural member having local deformation actuators. As will become apparent from the following summary, the present invention satisfies this need.